The development of an effective and safe vaccine is an essential component in the prevention and control of AIDS. Since most of the new HIV infections are acquired either through sexual activity, or to a lesser extent intravenously among injecting drug abusers, a vaccine will need to stimulate both the circulatory and mucosal immune responses to provide maximal protection against HIV infection and transmission. A major inductive site for mucosal immunity in humans is the nasal respiratory epithelium. In previous studies from our laboratory as well as others, the use of RNA vectors (replicons) based on poliovirus or Semliki Forest Virus (SFV) have established the potential of RNA immunization. We have found that RNA immunization with poliovirus replicons followed by intranasal inoculation with recombinant protein resulted in the production of antibodies in both the serum and secretions. Based on these studies, we propose that immunization with RNA vectors in the periphery, followed by booster intranasal immunization with recombinant proteins could be an effective way to stimulate both systemic and mucosal immunity. The goal of this R21 proposal then is to further develop this novel prime-boost vaccine approach. The following Specific Aims are proposed: Specific Aim 1: To determine if poliovirus or SFV based replicons are more effective in priming for induction of circulatory and mucosal response following nasal boost with recombinant antigen. We will compare the efficacy of poliovirus or SFV replicons that encode gag or envelope for the capacity to prime both the systemic and mucosal immune response when given via intramuscular immunization. Specific Aim 2: To determine if prime-boost with different HIV-1 clades will broaden the immune response. RNA replicons (polio or SFV) encoding clade C HIV-1 gag and envelope will be used for intramuscular immunization. We will then compare the immune response by intranasal immunization with recombinant clade B virus gag and envelope with that induced by intranasal immunization with HIV-1 clade C gag and envelope. The results of these studies should provide essential pre-clinical information as to the efficacy of the RNA prime/recombinant antigen boost vaccine protocol as a means to generate both systemic and mucosal immune responses to HIV-1.